Protein may help the eyes tell time.

The light-sensitive skin cells of the African horned frog may have resolved a lingering mystery about the internal biological clock that all mammals possess, suggests a new study.

This physiological timepiece, whose period is almost exactly a day, governs many bodily rhythms. To synchronize with the day-night cycle of the outside world, the mammalian biological clock adjusts itself daily by sensing sunlight and other stimuli.

How the clock detects illumination has long puzzled scientists. They know that human eyes perceive illumination and convey a signal to the portion of the brain where the clock resides. Yet this link doesn't appear to involve rods and cones, the eye's cells used to see. In fact, scientists have created mice lacking rods and cones, and the animals still shift their biological clocks in response to light.

"We thought we knew what the eye did. What we now know is that the eye is doing two quite separate tasks. It's grabbing light to construct a visual image, yet there's a separate pathway used for grabbing light for biological time," says Russell G. Foster of Imperial College in London, who led the research group that made the mice without rods and cones.

In 1998, some biologists suggested that cryptochromes, light-sensitive proteins first found in plants, were used by the human eye to gather light for the biological clock rather than the light-sensing opsin molecules in rods and cones (SN: 7/11/98, p. 24). Enthusiasm for that idea has since faded, partly because mice lacking the cryptochromes have clocks that remain light-sensitive.

A new nominee for the elusive photoreceptor is melanopsin. Mark D. Rollag and Iguacio Provencio of the Uniformed Services University of the Health Sciences in Bethesda, Md., and their colleagues originally discovered this member of the opsin family in the skin, eyes, and brains of frogs. In the Jan. 15 JOURNAL OF NEUROSCIENCE, they report finding the human version of the protein and reveal that melanopsin is made by cells in the eye's inner retina. In contrast, the eye's rods and cones are in the outer retina.

On the basis of those findings and a few other pieces of evidence, the scientists argue that melanopsin may be the photoreceptor for the human biological clock. Other researchers cautiously agree.

"Melanopsin is really intriguing. I think it's the best candidate we have," says Foster. "We need a few more experiments to absolutely nail it."